Email marketing tools - advise

Estimated duration: not set
Description: I am looking for advise on what is the best email marketing tool for us to use given the below.
We have a few busy sites and will get anything between 30-50 leads per day and some days over 100. We have our own site back end for lead management etc but need to plug in a good email marketing tool for auto responders. We will collect all the lead data on our site and landing pages. We would like to set up series of auto responders based on the readers actions. We have over 6,000 for one site and over 10,000 for the other site. Ideally we use the same account for both, but perhaps need to be separate as they are different businesses. I would like advise on which system to use. We dont use double opt-in but all are signing up for a brochure or to get a book or similar, there is no spam or bought leads, all very clean. In our ideal scenario we have an API to handshake with the email/autoresponder tools so that the base is kept up to date with the different lists.
At this stage I am looking for advise on this as a project. We then need help in setting it up, note we will do the programming our side, will need the rest setup and advise on doing so.
Extra notes: we would like the advise asap, hoping to set it up for end August.